1. Field of the Invention
The present invention relates to an apparatus for depositing a film onto the surface of a substrate by sputtering, and, more particularly, to a long distance sputtering apparatus in which the target and the substrate are separated by a larger distance than usual.
2. Description of the Related Art
A sputtering apparatus is widely used in the film deposition process of, for example, large-scale integrated circuits (LSIs), liquid crystal displays (LCDs), or information recording disks. FIG. 6 is a front cross section of a typical conventional sputtering apparatus. In such apparatus, a target 2, a magnetron electric discharge electrode 3, and a substrate holder 5 for holding a substrate 50 are provided in a vacuum vessel 1 provided with a gas-introducing mechanism 4 and an exhaust system 11.
Achieving improvements in step coverage, particularly in bottom coverage, is becoming very important in the field of semiconductor devices where the semiconductor devices are becoming increasingly densely integrated.
Step coverage refers to how well holes or grooves formed in the surface of a substrate, or a stepped section, are covered with a film. It is a term, including bottom coverage, that indicates the ratio of the film thickness at the bottom of a hole, groove, or stepped section to the film thickness at the top of a hole, groove, or stepped section.
As wiring becomes finer, the wiring width becomes narrower, on the one hand, and the height of the step (or depth of a hole or groove) becomes larger on the other, so that a sufficiently thick film can no longer be deposited at the bottom section of a deep hole or groove. For example, when a deposited barrier film is in a contact hole or through hole in the surface of a substrate by sputtering, and the barrier film deposited is not thick enough, mutual diffusion of atoms occurs between layers, resulting in junction leakage that may be fatal to the semiconductor devices.
Conventional sputtering apparatuses can only be used to deposit a film for a hole or groove having a width of up to 0.7 .mu.m and an aspect ratio (equal to the height of the hole or the groove/width of the hole or groove) of up to about 1 to 2. This is because in conventional apparatuses, such as the one shown in FIG. 6, there are a large number of sputtering particles that are allowed to be obliquely incident to the substrate 50, so that only a small number of sputtering particles can reach the bottom of the hole or groove. This has been a technical obstacle to producing an integrated circuit of 16 megabits or more. FIG. 7 is a cross sectional profile of a film deposited by the conventional sputtering apparatus of FIG. 6. As can be seen from FIG. 7, when conventional sputtering apparatuses are used, a film is deposited only at the bottom section of a hole or groove 500 in a substrate 50, so that bottom coverage is very low (expressed as b/a, with "a" denoting the thickness of the film at the top and "b" denoting the thickness of the film at the bottom).
Collimation sputtering is known as a method for increasing bottom coverage by increasing the number of sputtering particles that are allowed to be vertically incident to the substrate.
FIG. 8 is a front cross sectional view of a conventional sputtering apparatus that performs collimation sputtering. The sputtering apparatus of FIG. 8 has a collimation filter 7 disposed between the target 2 and the substrate 50. There are collimation filters 7 in which holes are arranged in a lattice structure, and those having a plurality of concentrically disposed ring-shaped plates. The collimation filter 7 passes only those particles travelling perpendicular to the surface of the substrate.
Collimation sputtering has a problem that when sputtering is continued for a long period of time, the cross sectional area of the passage in the collimation filter 7 becomes smaller because the sputtering particles stick to and accumulate on the collimation filter 7. A smaller cross-sectional area passage allows less sputtering particles to pass through the collimation filter, giving rise to the problem of a reduced deposition rate. This problem is considered fatal for a mass-produced apparatus. In addition, the collimation filter 7 requires maintenance such as replacement or cleaning, so that the apparatus cannot provide sufficient productivity for mass production.
Long distance sputtering has been recently drawing attention as a method that makes it possible to overcome the problems occurred in collimation sputtering. FIG. 9(a) is a front cross sectional view of a conventional long distance sputtering apparatus.
As can be seen from FIG. 9(a), in the sputtering apparatus the target 2 and the substrate 50 are separated by a larger distance than usual. (The distance between the target and the substrate is hereinafter referred to as the "IT/S distance"). For a longer T/S distance only those sputtering particles, among the sputtering particles ejected from the target 2, that travel nearly perpendicular to the surface of the substrate 50 reach the substrate 50. Those particles that travel obliquely to the surface of the substrate 50 move toward the walls of the vacuum vessel 1, as illustrated in FIG. 9(a). Accordingly, there is a relative increase in the number of sputtering particles that are allowed to be vertically incident to the surface of the substrate 50, resulting in significantly increased bottom coverage.
In the sputtering apparatus of FIG. 9(a), a shield 6 is provided inside a vacuum vessel 1 in order to reduce the number of sputtered particles sticking to the walls of the vessel 1. The shield 6 has gas-introducing holes 41 through which sputtering gas is introduced by means of a gas-introducing mechanism 4. As illustrated in FIG. 9(a), each gas-introducing hole 41 is located near the substrate holder 5, and at about 30 mm from the substrate 50.
The shield 6 has exhaust holes 42, provided near the gas-introducing holes 41, for discharging sputtering gas. More specifically, the exhaust holes 42 are located above and below the gas-introducing holes 41, where above and below are defined in the specification with respect to a line connecting the substrate 50 and the target 2.
The sputtering gas is also discharged through the space between each shield 6 and the target 2.
When the gas-introducing holes 41 and the exhaust holes 42 are disposed in this way, the pressure at locations toward the target 2 is low and the pressure at locations toward the substrate 50 is high, as can be seen by the straight line 40 indicating the pressure distribution of the sputtering gas in FIG. 9(b), as measured from the target 2 to the substrate 50.
In the above-described long distance sputtering apparatus, the T/S distance is increased and the sputtering gas pressure is decreased so that a greater number of sputtering particles are allowed to be vertically incident to the substrate, without scattering of the sputtering particles.